The present invention relates to macrocomposite bodies comprising a metal or metal matrix composite substrate featuring a coating or surface layer on at least a portion thereof. In particular, the present invention relates to such surface layers or coatings whose difference in such physical properties as hardness and abrasiveness relative to the substrate increase the usefulness of the macrocomposite bodies and ease of shaping same.
Composite products comprising a metal matrix and a strengthening or reinforcing phase such as ceramic particulates, whiskers, fibers or the like, show great promise for a variety of applications because they combine some of the stiffness and wear resistance of the reinforcing phase with the ductility and toughness of the metal matrix. Generally, a metal matrix composite (xe2x80x9cMMCxe2x80x9d) will show an improvement in such properties as strength, stiffness, contact wear resistance and elevated temperature strength retention relative to the matrix metal in monolithic form, but the degree to which any given property may be improved depends largely on the specific constituents, their volume or weight fraction, and how they are processed in forming the composite. In some instances, the composite also may be lighter in weight than the matrix metal per se. Aluminum matrix composites reinforced with ceramics such as silicon carbide in particulate, platelet, or whisker form, for example, are of interest because of their high stiffness, wear resistance and high temperature strength relative to aluminum.
U.S. Pat. No. 4,600,481 to Sane et al. discloses an electrolytic aluminum production cell component comprising a preformed matrix based on various ceramic materials including aluminum nitride, the preformed matrix having voids extending throughout its structure, the voids in the preformed matrix structure being filled or substantially filled with aluminum in intimate contact with the matrix usually in wetting contact whereby the matrix will remain permanently filled with molten aluminum in the conditions of use of the cell component. According to Sane et al. once the materials have been wetted by molten aluminum the wetting contact is maintained even at lower temperatures and under an atmosphere in which wetting could not initially be established. Further, by having the aluminum in wetting contact with the matrix retention of the aluminum in the structure is assisted and a protective film of aluminum forms at the surface protecting the structure from corrosion.
The metal matrix composite materials systems which presently command the greatest interest and the greatest share of the market are those having silicon carbide or aluminum oxide particulates several microns to tens of microns in size reinforcing a matrix predominantly of aluminum. Such materials systems combine good performance in terms of physical properties with relatively low cost. Such MMC""s can be rather abrasive, however, which can create wear problems in those applications where the MMC component is in sliding contact with an unreinforced metal component. Even in those applications where the contact is not in sliding contact, vibrations have the potential to cause fretting wear of the contacting parts. Moreover, most components require some amount of machining. Although xe2x80x9cexoticxe2x80x9d forms of machining such as water jet or laser machining are available, the machining infrastructure is still based on xe2x80x9ctraditionalxe2x80x9d machining using cutting or grinding tools. Further, sometimes only traditional machining can be used to machine certain geometries or provide a particular surface finish. The problem with the above-identified popular MMC material systems is that they tend to be difficult to machine at least with traditional cutting tools. Specifically, not only can machining stock not be removed quickly, the cutting tools become dull extremely rapidly. Whether the tools can be resharpened or must be disposed of a cost is imposed in terms of down-time to change or resharpen tools.
U.S. Pat. No. 5,511,603 to Brown et al. discloses a machinable metal matrix composite material. The inventors state that small sized particles for the reinforcement phase, no greater than about three microns in diameter and preferably less than one in conjunction with relatively low particle loading, and a substantially uniform distribution of ceramic particles in a sintered preform are all important for achieving a machinable composite material. Such metal matrix composites suffer, however, from the expense of such ultra-fine powders, the relative difficulty encountered in distributing them uniformly throughout a preform and infiltrating such preforms expeditiously.
Commonly owned U.S. Pat. No. 4,828,008 to White et al. teaches a technique for producing a metal matrix composite body by a spontaneous infiltration process. According to the White et al. invention, a permeable mass of ceramic filler material may be infiltrated by a molten aluminum alloy containing at least 1 weight percent magnesium in the presence of a gas comprising from about 10 to 100 volume percent nitrogen without the requirement for pressure or vacuum, whether externally applied or internally created. In one embodiment of the White et al. invention, the formed metal matrix composite body is provided with an aluminum nitride skin or surface. Specifically, if the supply of molten aluminum alloy matrix metal becomes exhausted before complete infiltration of the permeable ceramic filler material, an aluminum nitride layer or zone may form on or along the outer surface of the metal matrix composite. Also, an aluminum nitride skin can be formed at the exterior surface of the permeable mass of ceramic filler material by prolonging the process conditions. In particular, once infiltration of the permeable ceramic material is substantially complete if the infiltrated ceramic material is further exposed to the nitrogenous atmosphere at substantially the same temperature at which infiltration occurred, the molten aluminum at the exposed surface will nitride. The degree of nitridation can be controlled and may be formed as either a continuous phase or discontinuous phase in the skin layer.
Commonly owned U.S. Pat. No. 5,040,588 to Newkirk et al. teaches the production of macrocomposite bodies comprising one or more metal matrix composite bodies bonded to one or more second bodies. The second body may comprise ceramic, metal or composite bodies of ceramic and metal. In a preferred embodiment of the invention, a permeable mass or preform is placed in contact with the second body. A molten matrix metal is caused to infiltrate the permeable mass or preform up to the second body, the infiltrated mass or preform thereby becoming a metal matrix composite body. Upon solidifying the matrix metal, the formed MMC remains bonded to the second body.
Commonly owned U.S. Pat. No. 5,020,584 to Aghajanian et al. teaches the addition of matrix metal in powdered form to a permeable mass to filler material or a preform. The presence of powdered matrix metal in the preform or filler material reduces the relative volume fraction of filler material to matrix metal.
The present invention provides a technique for producing metal matrix composite bodies having surfaces which are easier to machine and are less abrasive in use. Machinability and wear compatibility (low abrasiveness) are important considerations in producing metal matrix composite bodies. Often, metal matrix composite bodies lack these important characteristics in part because hard ceramic materials, such as silicon carbide, are used as the reinforcement phase. The present invention addresses these problems by providing a surface layer on the MMC material which has the desired characteristics. Specifically, the surface layer is itself a MMC material comprising aluminum nitride.
To make such a metal matrix composite body, for example, a layer is provided to a permeable mass or preform at the surface or surfaces where a reduced volumetric loading of ceramic filler material relative to the permeable mass or preform is desired. The reduced loading may be achieved by incorporating a fugitive material into the layer. In a preferred embodiment, the fugitive material comprises a fugitive metal. In this embodiment, the presence of ceramic filler material in the layer is optional. The layer as applied is permeable and may be applied using most preforming techniques. A particularly preferred technique for applying the permeable coating layer is to prepare a slurry comprising the coating materials. and apply the slurry by processes such as painting, spraying, dip coating, stuccoing, etc.
Molten matrix metal is caused to infiltrate the permeable mass or preform and the coating layer to produce a macrocomposite body comprising a metal matrix composite substrate defined by the infiltrated permeable mass or preform and a metal matrix composite surface layer metallurgically bonded to the substrate. The metal matrix composite surface layer is machinable and distinguishable from the metal matrix composite substrate material. When the infiltration process is a spontaneous infiltration featuring a matrix metal comprising aluminum and an infiltrating atmosphere or an infiltration enhancer comprising nitrogen, aluminum nitride forms as a by-product, both in the substrate and in the coating layer. Because of this in-situ formed aluminum nitride, it is possible to apply a permeable coating which prior to infiltration contains no ceramic reinforcement (e.g., filler) materials. and end up with a MMC material containing ceramic material after matrix metal infiltration, the in-situ formed aluminum nitride being the ceramic reinforcement.
The present xe2x80x9cfugitive material coating techniquexe2x80x9d permits a thicker machinable layer to be produced than could otherwise be produced by permitting molten aluminum to xe2x80x9cover-infiltratexe2x80x9d a permeable mass or preform to form an aluminum nitride-bearing xe2x80x9cskinxe2x80x9d on the surface of the formed composite. The thickness of the surface layer may become an issue because if the layer is to be machined, adequate machining stock must be provided.
Even when the surface layer contains no ceramic filler material added to the permeable surface coating composition, the as-infiltrated surface layer is a metal matrix composite material when in-situ formed aluminum nitride is present. Particularly when the as-infiltrated surface layer consists essentially of aluminum alloy and aluminum nitride, such surface layer is likely to be substantially more machinable and less abrasive than the metal matrix composite substrate to which it is attached.
The techniques of the present invention may be used to produce an MMC surface layer on an already-formed MMC substrate body. Specifically, a MMC substrate may be coated on one or more surfaces with the permeable coating layer comprising at least one fugitive material. If the MMC substrate body was not produced by a spontaneous infiltration process, and if it is desired that the MMC coating layer be formed by spontaneous infiltration. it may be necessary to provide an infiltration enhancer or infiltration enhancer precursor to the system at some point during processing. The matrix metal which infiltrates the coating layer could be supplied by the MMC substrate body, or from a different source.
Likewise, the above technique may be applied to substrates of unreinforced metals to yield MMC coated metals in which the MMC surface coating is wear compatible and machinable.
Definitions
xe2x80x9cAluminumxe2x80x9d, as used herein, means and includes essentially pure metal (e.g., a relatively pure, commercially available unalloyed aluminum) or other grades of metal and metal alloys such as the commercially available metals having impurities and/or alloying constituents such as iron, silicon, copper, magnesium, manganese, chromium, zinc, etc., therein. An aluminum alloy for purposes of this definition is an alloy or intermetallic compound in which aluminum is the major constituent.
xe2x80x9cBalance Non-Oxidizing Gasxe2x80x9d, as used herein, means that any gas present in addition to the primary gas comprising the infiltrating atmosphere, is either an inert gas or a reducing gas which is substantially non-reactive with the matrix metal under the process conditions. Any oxidizing gas which may be present as an impurity in the gas(es) used should be insufficient to oxidize the matrix metal to any substantial extent under the process conditions.
xe2x80x9cBarrierxe2x80x9d or xe2x80x9cbarrier meansxe2x80x9d, as used herein, means any suitable means which interferes, inhibits, prevents or terminates the migration, movement, or the like, of molten matrix metal beyond a surface boundary of a permeable mass of filler material or preform, where such surface boundary is defined by said barrier means. Suitable barrier means may be any such material, compound, element, composition, or the like, which, under the process conditions, maintains some integrity and is not substantially volatile (i.e., the barrier material does not volatilize to such an extent that it is rendered non-functional as a barrier).
Further, suitable xe2x80x9cbarrier meansxe2x80x9d includes materials which are substantially non-wettable by the migrating molten matrix metal under the process conditions employed. A barrier of this type appears to exhibit substantially little or no affinity for the molten matrix metal, and movement beyond the defined surface boundary of the mass of filler material or preform is prevented or inhibited by the barrier means. The barrier reduces any final machining or grinding that may be required and defines at least a portion of the surface of the resulting metal matrix composite product. The barrier may in certain cases be permeable or porous, or rendered permeable by, for example, drilling holes or puncturing the barrier, to permit gas to contact the molten matrix metal.
xe2x80x9cCarcassxe2x80x9d or xe2x80x9cCarcass of Matrix Metalxe2x80x9d, as used herein, refers to any of the original body of matrix metal remaining which has not been consumed during formation of the metal matrix composite body, and typically, if allowed to cool, remains in at least partial contact with the metal matrix composite body which has been formed. It should be understood that the carcass may also include a second or foreign metal therein.
xe2x80x9cFillerxe2x80x9d, as used herein, is intended to include either single constituents or mixtures of constituents which are substantially non-reactive with and/or of limited solubility in the matrix metal and may be single or multi-phase. Fillers may be provided in a wide variety of forms. such as powders, flakes, platelets, microspheres, whiskers, bubbles, etc., and may be either dense or porous. xe2x80x9cFillerxe2x80x9d may also include ceramic fillers, such as alumina or silicon carbide as fibers, chopped fibers, particulates, whiskers, bubbles, spheres, fiber mats, or the like, and ceramic-coated fillers such as carbon fibers coated with alumina or silicon carbide to protect the carbon from attack, for example, by a molten aluminum parent metal. Fillers may also include metals.
xe2x80x9cFugitive Materialxe2x80x9d, as used herein. means a material which permits and results in a reduced volumetric loading of ceramic filler material in the MMC surface layer by displacing such filler material in the permeable surface coating.
xe2x80x9cFugitive Metalxe2x80x9d, as used herein, means a fugitive material which possesses at least one property characteristic of metals and which is capable of reaction with or substantial dissolution of or into the matrix metal. Fugitive metals include for example, the semimetals silicon, germanium, boron, and arsenic but exclude carbon.
xe2x80x9cInfiltrationxe2x80x9d, as used herein, means the bulk transport of matrix metal into a permeable mass or permeable surface layer, with or without pressure or vacuum assist.
xe2x80x9cInfiltrating Atmospherexe2x80x9d, as used herein, means that atmosphere which is present which interacts with the matrix metal and/or preform (or filler material) and/or infiltration enhancer precursor and/or infiltration enhancer and permits or enhances spontaneous infiltration of the matrix metal to occur.
xe2x80x9cInfiltration Enhancerxe2x80x9d, as used herein, means a material which promotes or assists in the spontaneous infiltration of a matrix metal into a filler material or preform. An infiltration enhancer may be formed from. for example, a reaction of an infiltration enhancer precursor with an infiltrating atmosphere to form (1) a gaseous species and/or (2) a reaction product of the infiltration enhancer precursor and the infiltrating atmosphere and/or (3) a reaction product of the infiltration enhancer precursor and the filler material or preform. Moreover, the infiltration enhancer may be supplied directly to at least one of the preform, and/or matrix metal, and/or infiltrating atmosphere and function in a substantially similar manner to an infiltration enhancer which has formed as a reaction between an infiltration enhancer precursor and another species. Ultimately, at least during the spontaneous infiltration, the infiltration enhancer should be located in at least a portion of the filler material or preform to achieve spontaneous infiltration.
xe2x80x9cInfiltration Enhancer Precursorxe2x80x9d or xe2x80x9cPrecursor to the Infiltration Enhancerxe2x80x9d, as used herein, means a material which when used in combination with the matrix metal, preform and/or infiltrating atmosphere forms an infiltration enhancer which induces or assists the matrix metal to spontaneously infiltrate the filler material or preform. Without wishing to be bound by any particular theory or explanation, it appears as though it may be necessary for the precursor to the infiltration enhancer to be capable of being positioned, located or transportable to a location which permits the infiltration enhancer precursor to interact with the infiltrating atmosphere and/or the preform or filler material and/or metal. For example, in some matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems, it is desirable for the infiltration enhancer precursor to volatilize at, near, or in some cases, even somewhat above the temperature at which the matrix metal becomes molten. Such volatilization may lead to: (1) a reaction of the infiltration enhancer precursor with the infiltrating atmosphere to form a gaseous species which enhances wetting of the filler material or preform by the matrix metal; and/or (2) a reaction of the infiltration enhancer precursor with the infiltrating atmosphere to form a solid, liquid or gaseous infiltration enhancer in at least a portion of the filler material or preform which enhances wetting; and/or (3) a reaction of the infiltration enhancer precursor within the filler material or preform which forms a solid, liquid or gaseous infiltration enhancer in at least a portion of the filler material or preform which enhances wetting.
xe2x80x9cMatrix Metalxe2x80x9d or xe2x80x9cMatrix Metal Alloyxe2x80x9d, as used herein, means that metal which is utilized to form a metal matrix composite (e.g., before infiltration) and/or that metal which is intermingled with a filler material to form a metal matrix composite body (e.g., after infiltration). When a specified metal is mentioned as the matrix metal, it should be understood that such matrix metal includes that metal as an essentially pure metal, a commercially available metal having impurities and/or alloying constituents therein, an intermetallic compound or an alloy in which that metal is the major or predominant constituent.
xe2x80x9cMatrix Metal/Infiltration Enhancer Precursor/Infiltrating Atmosphere Systemxe2x80x9d or xe2x80x9cSpontaneous Systemxe2x80x9d, as used herein, refers to that combination of materials which exhibit spontaneous infiltration into a preform or filler material. It should be understood that whenever a xe2x80x9c/xe2x80x9d appears between an exemplary matrix metal, infiltration enhancer precursor and infiltrating atmosphere that the xe2x80x9c/xe2x80x9d is used to designate a system or combination of materials which, when combined in a particular manner, exhibits spontaneous infiltration into a preform or filler material.
xe2x80x9cMetal Matrix Compositexe2x80x9d or xe2x80x9cMMCxe2x80x9d, as used herein, means a material comprising a two- or three-dimensionally interconnected alloy or matrix metal which has embedded a preform or filler material. The matrix metal may include various alloying elements to provide specifically desired mechanical and physical properties in the resulting composite.
A Metal xe2x80x9cDifferentxe2x80x9d from the Matrix Metal means a metal which does not contain, as a primary constituent, the same metal as the matrix metal (e.g., if the primary constituent of the matrix metal is aluminum, the xe2x80x9cdifferentxe2x80x9d metal could have a primary constituent of, for example nickel).
xe2x80x9cPermeable Surface Layerxe2x80x9d or xe2x80x9cPermeable Coating,xe2x80x9d as used herein. means a porous mass featuring at least one fugitive material which is applied to one or more surfaces of a body with the intent or objective that such mass will be infiltrated with molten metal to form an MMC surface layer. If a filler material is not present in the permeable surface layer, at least one filler material must be formed in situ during infiltration.
xe2x80x9cPreformxe2x80x9d or xe2x80x9cPermeable Preformxe2x80x9d, as used herein, means a porous mass of filler or filler material which is manufactured with at least one surface boundary which essentially defines a boundary for infiltrating matrix metal, such mass retaining sufficient shape integrity and green strength to provide dimensional fidelity prior to being infiltrated by the matrix metal. The mass should be sufficiently porous to accommodate spontaneous infiltration of the matrix metal thereinto. A preform typically comprises a bonded array or arrangement of filler, either homogeneous or heterogeneous. and may be comprised of any suitable material (e.g., ceramic and/or metal particulates, powders, fibers, whiskers, etc., and any combination thereof). A preform may exist either singularly or as an assemblage.
xe2x80x9cReservoirxe2x80x9d, as used herein, means a separate body of matrix metal positioned relative to a mass of filler or a preform so that, when the metal is molten, it may flow to replenish, or in some cases to initially provide and subsequently replenish, that portion, segment or source of matrix metal which is in contact with the filler or preform.
xe2x80x9cSpontaneous Infiltrationxe2x80x9d, as used herein, means the infiltration of matrix metal into the permeable mass of filler or preform occurs without requirement for the application of pressure or vacuum (whether externally applied or internally created).
xe2x80x9cSubstratexe2x80x9d or xe2x80x9cMMC Substratexe2x80x9d, as used herein, means the body to which the surface layer is applied, and which defines the basic size and shape of the desired article.
xe2x80x9cSurface Layerxe2x80x9d or xe2x80x9cMMC Surface Layerxe2x80x9d, as used herein, means the MMC material deposited or formed on at least a portion of at least one surface of the substrate and having reduced abrasiveness and/or enhanced machinability with respect to an MMC substrate.